Retort from Scrooge is sweet (6)
I believe the answer is:
humbug
'is sweet' is the definition.
(boiled sweet)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for humbug that I've seen before include "talk that's deceptive" , "Peppermint sweet - deception" , "Pretentious talk or boiled sweet" , "It's stuff and nonsense about the sweet" , "Baloney" .)